Microparticles, in particular, nanoparticles whose particle diameter is less than 1 μm (nanosized fine particles) exhibit new characteristics that are different from those of particles, and therefore, a new method for industrially producing nanoparticles is keenly urged.
The applicant of the present invention provided a method for separating nanoparticles by stirring and instantaneously uniform mixing of a plurality of fluids in a microscopic flow path, based on a totally new concept of “microchemical process technology” that solved subjects and problems in the conventional “microchemical process technology”, more specifically by using the principle based on the apparatus shown in Patent Document 1, which was filed by the applicant of the present invention (Patent Document 2). This apparatus utilizes the principle of a mechanical seal, wherein a forced thin film of a fluid to be processed is formed between processing surfaces capable of approaching to and separating from each other and being displaced relative to the other thereby supplying the fluid to be processed into between the rotating processing surfaces, whereby realizing a minute distance between the processing surfaces by the pressure balance between the supply pressure of the fluid and the pressure exerted between the rotating processing surfaces. In a method used before the method based on the foregoing principle, the distance between the processing surfaces was controlled mechanically or the like; therefore heat generated by rotation, deformation caused by it, axial run-out, or the like could not be absorbed, and thus it was practically impossible to make the distance between the processing surfaces minute, at least to the level of 10 μm or less. In other words, by using the principle of the apparatus shown in Patent Document 1, separation of nanoparticles by an instantaneous chemical reaction, physical chemical reaction, or the like in a microscopic flow path could be realized; and as a result of the extensive investigation of the inventors of the present application, an instantaneous stirring/mixing/reaction/separation in a microscopic flow path with the size of not only 1 mm or less but also surprisingly 0.1 μm to 10 μm could be realized.
By using the method for producing pigment nanoparticles proposed in Patent Document 2, nanosized pigments can be produced at a low cost and a low energy; nevertheless, a method for producing nanoparticles having a further smaller particle diameter and being capable of further easier re-dispersion has been wanted. In addition, control of a particle size thereof has been difficult. Accordingly, a method for producing nanoparticles having an intended particle diameter and being capable of further easier re-dispersion has been wanted.
A diketopyrrolopyrrole pigment, classified into, in accordance with the Color Index number, Pigment Orange 71, Pigment Orange 73, Pigment Red 254, Pigment Red 255, Pigment Red 264, and so on, is an organic pigment which is excellent especially in durability such as weather resistance, light resistance, and heat resistance.
The diketopyrrolopyrrole pigment, known to generally have an α-type and a β-type in its crystal structure, is frequently used mainly for a red color filter centered on Pigment Red 254. It is natural that further finer particles are wanted; and on top of it, a method for producing α-type diketopyrrolopyrrole pigment nanoparticles having further improved solvent resistance and durability is wanted, because these particles have a problem such as crystal growth in a solvent, during coating thereof, and the like, depending on the crystal type and the crystallinity thereof.
As to the method for producing the α-type diketopyrrolopyrrole pigment nanoparticles, a method comprising two steps, namely a first step in which diketopyrrolopyrrole pigment particles are separated and a second step in which a crystal type of the separated diketopyrrolopyrrole pigment particles is transformed in an organic solvent being capable of transforming the crystal type thereof, has been known, as described in Patent Document 3 or Patent Document 4. However, because crystal growth of the pigment particles occurs inevitably in the step of crystal type transformation, it is difficult to keep a particle diameter fine; and thus, it is difficult to provide nanoparticles having a uniform particle diameter and a spherical shape.
In addition to the above-mentioned method, a so-called solvent salt milling method, in which a mixture of an α-type and a β-type diketopyrrolopyrrole pigment is milled by wet grinding in an organic solvent by using an inorganic salt such as sodium chloride as a triturating agent, has been known, as described in Patent Document 5. In this method, crystal type transformation accompanied with crystal growth is effected in parallel with pulverization by milling.
However, when a method including the milling step as described in Patent Document 5 is used to control a crystal type of the pigment nanoparticles, there has been a problem that characteristics expected for the pigment nanoparticles, such as color tone, transparency, spectral property, and durability, are not expressed, because strong force acts on the pigment nanoparticles (crystal), as described in Patent Document 3.
The applicant of the present invention had provided a method for producing pigment nanoparticles wherein a pigment substance is separated in a thin film fluid passing through between processing surfaces arranged to be opposite to each other, as described in Patent Document 2; but the method to produce an α-type diketopyrrolopyrrole was not specifically disclosed therein. Accordingly, inventors of the present application attempted to separate the α-type diketopyrrolopyrrole in a thin film fluid containing a diketopyrrolopyrrole pigment solution containing at least one kind of diketopyrrolopyrrole pigments dissolved in a solvent and an alcohol solvent containing at least one kind of alcohol compound solvent by using the method for producing pigment nanoparticles described in Patent Document 2. However, in this method, it was difficult to separate the α-type diketopyrrolopyrrole pigment nanoparticles having high crystallinity, and in addition, there was a case that a mixture of the α-type with other crystal type (β-type) was obtained; and thus, the diketopyrrolopyrrole pigment nanoparticles substantially comprising only the α-type could not be produced efficiently.    Patent Document 1: JP-A-2004-49957    Patent Document 2: International Patent Laid-Open Publication No. 2009/008388    Patent Document 3: International Patent Laid-Open Publication No. 2008/044519    Patent Document 4: International Patent Laid-Open Publication No. 2009/081930    Patent Document 5: JP-A-2008-24873